1. Field of the Invention
The present invention relates to a radiation measurement system which monitors radiation in nuclear facilities, in nuclear waste material reprocessing facilities, and the like.
2. Description of the Related Art
A conventional radiation measurement system includes: a first sample vessel of large volume, in which a sample serving as a radiation measurement object is made to flow; a second sample vessel of small volume; a radiation detector which detects a gamma ray emitted from a radioactive material in the flowing sample; a shield which surrounds these components to shield the radiation detector from environmental radiation; and a measurement unit to which a detection signal output from the radiation detector is input, and which measures radiation. In the radiation measurement system, there is proposed a method in which arrangement is made in the order of the second sample vessel of small volume (high concentration measurement vessel), a collimator, the first sample vessel of large volume (low concentration measurement vessel), and the radiation detector in a face-to-face relation; the flow of the first and the second sample vessels are automatically switched by a solenoid valve at a predetermined radiation measurement value to measure radiation in a wide range (see Patent Document 1).
Furthermore, as for a conventional radiation measurement system, there is proposed a device in which a movable collimator having an inner diameter equivalent to a radiation detector is arranged between a sample vessel and the radiation detector and a shield which incorporates and surrounds these components to shield the radiation detector from environmental radiation is provided. In the radiation measurement system, the movable collimator is made to be close to the radiation detector so as not to disturb measurement during a low range; and the movable collimator is made to be close to the sample vessel in order to narrow down the number of radiation incident on the radiation detector per unit time during a high range to measure radiation in a wide range (see Patent Document 2).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2001-153956 (FIG. 1)
[Patent Document 2] Japanese Unexamined Patent Publication No. 2005-9890 (FIG. 1, FIG. 2)
The conventional radiation measurement systems are thus configured; and accordingly, a problem exists in that when the flow is switched from the large first sample vessel to the small second sample vessel, the method of switching the sample vessels performs purge of the large first sample vessel and therefore missing measurement is present therebetween. Furthermore, in the movable collimator system, a problem exists in that when it is tried to enlarge a measurement range, a movement distance of the movable collimator needs to be increased and accordingly the shield increases in size. Besides, measurement result is unstable during the movement of the movable collimator and thus there is a problem in stability and responsiveness of a high range area required for short-time response. Further, the movable collimator of heavy material is made to move and therefore a structure and a mechanism are complicated; and precision is required so that the accuracy of a stop position does not have an affect on a measurement error and therefore there is a problem in costs.